| Cytokinesis is the mechanical process by which one cell divides its cytoplasm and membrane material to become two. In eukaryotic cells, myosin II accumulates with actin at the cleavage furrow and is thought to provide the contractile force necessary for efficient cytokinesis. However, the importance of the actomyosin-based cytokinetic ring during division in Saccharomyces cerevisiae has been controversial. Using both deletion analysis and a conditional dominant negative construct we show that Myo1p, the single myosin II in S. cerevisiae, is essential for cytokinesis. Further, we show that a myo1Δ strain previously reported to grow nearly as well as wild type contains a single suppressor mutation that alleviates the severe cytokinesis defects of myo1Δ through the formation of multiple aberrant septa. Together, these results strongly suggest that the actomyosin ring is crucial for successful cytokinesis in budding yeast through guidance of septa formation.; A long-standing question in the field of cytokinesis has been how components of the cytokinetic ring localize to the site of cell division. In order to provide insight into the mechanism of myosin II recruitment, we have used a structure/function approach to define the minimal localization domain of Myo1p. Our data suggest that a novel structure, formed by a small region of the Myo1p tail, is both necessary and sufficient for localization to the division site. Additionally we have investigated the mechanism of actin ring assembly. We first show that actin filaments are required for cytokinesis in S. cerevisiae, and that the actin ring is a highly dynamic structure that undergoes constant turnover. Assembly of the actin ring requires the formin-like proteins and profilin but is not Arp2/3-mediated. Furthermore, the formin-dependent actin ring assembly pathway is regulated by the Rho-type GTPase Rho1p but not Cdc42p. Finally, we show that the formins are not required for localization of Cyk1p, an IQGAP-like protein previously shown to be required for actin ring formation, suggesting that the formin pathway and Cyk1p act synergistically but independently in assembly of the actin ring. |
